Potassium (K+) channels are ubiquitous proteins which are involved in the setting of the resting membrane potential as well as in the modulation of the electrical activity of cells. In excitable cells, K+ channels influence action potential waveforms, firing frequency, and neurotransmitter secretion (Rudy, B. (1988) Neuroscience, 25, 729-749; Hille, B. (1992) Ionic Channels of Excitable Membranes, 2nd Ed.). In non-excitable cells, they are involved in hormone secretion, cell volume regulation and potentially in cell proliferation and differentiation (Lewis et al. (1995) Annu. Rev. Immunol., 13, 623-653). Developments in electrophysiology have allowed the identification and the characterization of an astonishing variety of K+channels that differ in their biophysical properties, pharmacology, regulation and tissue distribution (Rudy, B. (1988) Neuroscience, 25, 729-749; Hille, B. (1992) Ionic Channels of Excitable Membranes, 2nd Ed.). More recently, cloning efforts have shed considerable light on the mechanisms that determine this functional diversity. Furthermore, analyses of structure-function relationships have provided an important set of data concerning the molecular basis of the biophysical properties (selectivity, gating, assembly) and the pharmacological properties of cloned K+ channels.
Functional diversity of K+ channels arises mainly from the existence of a great number of genes coding for pore-forming subunits, as well as for other associated regulatory subunits. Two main structural families of pore-forming subunits have been identified. The first one consists of subunits with a conserved hydrophobic core containing six transmembrane domains (TMDs). These K+ channel cc subunits participate in the formation of outward rectifier voltage-gated (Kv) and Ca2+-dependent K+ channels. The fourth TMD contains repeated positive charges involved in the voltage gating of these channels and hence in their outward rectification (Logothetis et al. (1992) Neuron, 8, 531-540; Bezanilla et al. (1994) Biophys. J. 66, 1011-1021).
The second family of pore-forming subunits have only two TMDs. They are essential subunits of inward-rectifying (IRK), G-protein-coupled (GIRK) and ATP-sensitive (KATP) K+ channels. The inward rectification results from a voltage-dependent block by cytoplasmic Mg2+ and polyamines (Matsuda, H. (1991) Annu. Rev. Physiol., 53, 289-298). A conserved domain, called the P domain, is present in all members of both families (Pongs, O. (1993) J. Membr. Biol., 136, 1-8; Heginbotham et al. (1994) Biophys. J. 66,1061-1067; Mackinnon, R. (1995) Neuron, 14, 889-892; Pascual et al.,.(1995) Neuron., and 14, 1055-1063). This domain is an essential element of the aqueous K+ -selective pore. In both groups, the assembly of four subunits is necessary to form a functional K+ channel (Mackinnon, R. (1991) Nature, 350, 232-235; Yang et al., (1995) Neuron, 15, 1441-1447.
In both six TMD and two TMD pore-forming subunit families, different subunits coded by different genes can associate to form heterotetramers with new channel properties (Isacoff et al., (1990) Nature, 345, 530-534). A selective formation of heteropolymeric channels may allow each cell to develop the best K+ current repertoire suited to its function. Pore-forming xcex1 subunits of Kv channels are classified into different subfamilies according to their sequence similarity (Chandy et al. (1993) Trends Pharmacol. Sci., 14, 434). Tetramerization is believed to occur preferentially between members of each subgroup (Covarrubias et al. (1991) Neuron, 7, 763-773). The domain responsible for this selective association is localized in the N-terminal region and is conserved between members of the same subgroup. This domain is necessary for hetero- but not homomultimeric assembly within a subfamily and prevents co-assembly between subfamilies. Recently, pore-forming subunits with two TMDs were also shown to co-assemble to form heteropolymers (Duprat et al. (1995) Biochem. Biophys. Res. Commun., 212, 657-663. This heteropolymerization seems necessary to give functional GIRKs. IRKs are active as homopolymers but also form heteropolymers.
New structural types of K+ channels were identified recently in both humans and yeast. These channels have two P domains in their functional subunit instead of only one (Ketchum et al. (1995) Nature, 376, 690-695; Lesage et al. (1996) J. Biol. Chem, 271, 4183-4187; Lesage et al. (1996) EMBO J., 15, 1004-1011; Reid et al. (1996) Receptors Channels 4, 51-62). The human channel called TWIK-1, has four TMDs. TWIK-1 is expressed widely in human tissues and is particularly abundant in the heart and the brain. TWIK-1 currents are time independent and inwardly rectifying. These properties suggest that TWIK-1 channels are involved in the control of the background K+ membrane conductance (Lesage et al. (1996) EMBO J., 15, 1004-1011).
The present invention is based, at least in part, on the discovery of novel members of the TWIK (for Tandem of P domains in a Weak Inward rectifying K+ channel) family of potassium channels, referred to herein as TWIK-2, TWIK-3, TWIK-4, and TWIK-5 nucleic acid and protein molecules. The TWIK-2, TWIK-3, TWIK-4, and TWIK-5 molecules of the present invention are useful as targets for developing modulating agents to regulate a variety of cellular processes. Accordingly, in one aspect, this invention provides isolated nucleic acid molecules encoding TWIK proteins or biologically active portions thereof, as well as nucleic acid fragments suitable as primers or hybridization probes for the detection of TWIK-encoding nucleic acids.
In one embodiment, a TWIK nucleic acid molecule of the invention is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more identical to the nucleotide sequence (e.g., to the entire length of the nucleotide sequence) shown in SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:10, SEQ ID NO:12, or the nucleotide sequence of the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640, or a complement thereof.
In a preferred embodiment, the isolated nucleic acid molecule includes the nucleotide sequence shown SEQ ID NO:1 or 3, or a complement thereof. In another embodiment, the nucleic acid molecule includes SEQ ID NO:3 and nucleotides 1-9 of SEQ ID NO:1. In yet another embodiment, the nucleic acid molecule includes SEQ ID NO:3 and nucleotides 1507-3452 of SEQ ID NO:1. In another preferred embodiment, the nucleic acid molecule has the nucleotide sequence shown in SEQ ID NO:1 or 3. In another preferred embodiment, the nucleic acid molecule includes a fragment of at least 1644 nucleotides of the nucleotide sequence of SEQ ID NO:1, SEQ ID NO:3, or a complement thereof.
In another preferred embodiment, the isolated nucleic acid molecule includes the nucleotide sequence shown SEQ ID NO:4 or 6, or a complement thereof. In another embodiment, the nucleic acid molecule includes SEQ ID NO:6 and nucleotides 1-121 of SEQ ID NO:4. In yet another embodiment, the nucleic acid molecule includes SEQ ID NO:6 and nucleotides 1118-1575 of SEQ ID NO:4. In another preferred embodiment, the nucleic acid molecule has the nucleotide sequence shown in SEQ ID NO:4 or 6. In another preferred embodiment, the nucleic acid molecule includes a fragment of at least 369 nucleotides of the nucleotide sequence of SEQ ID NO:4, SEQ ID NO:6, or a complement thereof.
In another preferred embodiment, the isolated nucleic acid molecule includes the nucleotide sequence shown SEQ ID NO:7 or 9, or a complement thereof. In another embodiment, the nucleic acid molecule includes SEQ ID NO:9 and nucleotides 1-135 of SEQ ID NO:7. In yet another embodiment, the nucleic acid molecule includes SEQ ID NO:9 and nucleotides 1075-2287 of SEQ ID NO:7. In another preferred embodiment, the nucleic acid molecule has the nucleotide sequence shown in SEQ ID NO:7 or 9. In another preferred embodiment, the nucleic acid molecule includes a fragment of at least 537 nucleotides of the nucleotide sequence of SEQ ID NO:7, SEQ ID NO:9, or a complement thereof.
In yet another preferred embodiment, the isolated nucleic acid molecule includes the nucleotide sequence shown SEQ ID NO:10 or 12, or a complement thereof. In another embodiment, the nucleic acid molecule includes SEQ ID NO:12 and nucleotides 1-156 of SEQ ID NO:10. In yet another embodiment, the nucleic acid molecule includes SEQ ID NO:12 and nucleotides 1361-1506 of SEQ ID NO:10. In another preferred embodiment, the nucleic acid molecule has the nucleotide sequence shown in SEQ ID NO:10 or 12.
In another embodiment, a TWIK nucleic acid molecule includes a nucleotide sequence encoding a protein having an amino acid sequence sufficiently identical to the amino acid sequence of SEQ ID NO: 2, SEQ ID NO:5, SEQ ID NO:8, SEQ ID NO:11, SEQ ID NO:11, or an amino acid sequence encoded by the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640. In a preferred embodiment, a TWIK nucleic acid molecule includes a nucleotide sequence encoding a protein having an amino acid sequence at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or more identical to the amino acid sequence of SEQ ID NO: 2, SEQ ID NO:5, SEQ ID NO:8, or SEQ ID NO:11, or the amino acid sequence encoded by the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640.
In another preferred embodiment, an isolated nucleic acid molecule encodes the amino acid sequence of human TWIK-2, TWIK-3, TWIK4, or TWIK-5. In yet another preferred embodiment, the nucleic acid molecule includes a nucleotide sequence encoding a protein having the amino acid sequence of SEQ ID NO: 2, SEQ ID NO:5, SEQ ID NO:8, SEQ ID NO:11, or SEQ ID NO:11, or the amino acid sequence encoded by the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640. In yet another preferred embodiment, the nucleic acid molecule is at least 537 nucleotides in length and encodes a protein having a TWIK activity (as described herein).
Another embodiment of the invention features nucleic acid molecules, preferably TWIK nucleic acid molecules, which specifically detect TWIK nucleic acid molecules relative to nucleic acid molecules encoding non-TWIK proteins. For example, in one embodiment, such a nucleic acid molecule is at least 369, 400-450, 450-500, 500-550, 550-600, 600-650, 650-700, 700-750, 750-800 or more nucleotides in length and hybridizes under stringent conditions to a nucleic acid molecule comprising the nucleotide sequence shown in SEQ ID NO:4, the nucleotide sequence of the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640, or a complement thereof. In preferred embodiments, the nucleic acid molecules are at least 15 (e.g., contiguous) nucleotides in length and hybridize under stringent conditions to nucleotides 1-397, 586-670, 904-1111, or 1573-1575 of SEQ ID NO:4. In other preferred embodiments, the nucleic acid molecules comprise nucleotides 1-397, 586-670, 904-1111, or 1573-1575 of SEQ ID NO:4.
In another particularly preferred embodiment, the nucleic acid molecule comprises a fragment of at least 537, 550-600, 600-650, 650-700, 700-750, 750-800 or more nucleotides in length and hybridizes under stringent conditions to a nucleic acid molecule comprising the nucleotide sequence shown in SEQ ID NO:7, the nucleotide sequence of the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640, or a complement thereof. In preferred embodiments, the nucleic acid molecules are at least 15 (e.g., contiguous) nucleotides in length and hybridize under stringent conditions to nucleotides 1-164, 207-404, 1037-1789, 1818-1869, 1972-1985, or 2258-2287 of SEQ ID NO:7. In other preferred embodiments, the nucleic acid molecules include nucleotides 1-164, 207-404, 1037-1789, 1818-1869, 1972-1985, or 2258-2287 of SEQ ID NO:7.
In another particularly preferred embodiment, the nucleic acid molecule comprises a fragment of at least 550-600, 600-650, 650-700, 700-750, 750-800, 805, 850-900 or more nucleotides in length and hybridizes under stringent conditions to a nucleic acid molecule comprising the nucleotide sequence shown in SEQ ID NO:10, the nucleotide sequence of the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640, or a complement thereof.
In other preferred embodiments, the nucleic acid molecule encodes a naturally occurring allelic variant of a polypeptide comprising the amino acid sequence of SEQ ID NO:2, 5, 8, 11, or an amino acid sequence encoded by the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640, wherein the nucleic acid molecule hybridizes to a nucleic acid molecule comprising SEQ ID NO:1, 3, 4, 6, 7, 9, 10, or 12 under stringent conditions.
Another embodiment of the invention provides an isolated nucleic acid molecule which is antisense to a TWIK nucleic acid molecule, e.g., the coding strand of a TWIK nucleic acid molecule.
Another aspect of the invention provides a vector comprising a TWIK nucleic acid molecule. In certain embodiments, the vector is a recombinant expression vector. In another embodiment, the invention provides a host cell containing a vector of the invention. The invention also provides a method for producing a protein, preferably a TWIK protein, by culturing in a suitable medium, a host cell, e.g., a mammalian host cell such as a non-human mammalian cell, of the invention containing a recombinant expression vector, such that the protein is produced.
Another aspect of this invention features isolated or recombinant TWIK proteins and polypeptides. In one embodiment, the isolated protein, preferably a TWIK protein, includes at least one transmembrane domain. In another embodiment, the isolated protein, preferably a TWIK protein, includes at least one P-loop. In another embodiment, the isolated protein, preferably a TWIK protein, includes at least one transmembrane domain and at least one P-loop. In a preferred embodiment, the protein, preferably a TWIK protein, includes at least one transmembrane domain and at least one P-loop and has an amino acid sequence at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or more identical to the amino acid sequence of SEQ ID NO: 2, SEQ ID NO:5, SEQ ID NO:8, SEQ ID NO:11, SEQ ID NO:11 or the amino acid sequence encoded by the DNA insert of the plasmid deposited with ATCC as Accession Number PTA-1640. In another preferred embodiment, the protein, preferably a TWIK protein, includes at least one transmembrane domain and plays a role in generating an electrical potential across a plasma membrane, e.g., a neuronal plasma membrane or a muscle plasma membrane. In another preferred embodiment, the protein, preferably a TWIK protein, includes at least one P-loop and plays a role in generating an electrical potential across a plasma membrane, e.g., a neuronal plasma membrane or a muscle plasma membrane. In another preferred embodiment, the protein, preferably a TWIK protein, includes at least one transmembrane domain and at least one P-loop, and plays a role in generating an electrical potential across a plasma membrane, e.g., a neuronal plasma membrane or a muscle plasma membrane. In yet another preferred embodiment, the protein, preferably a TWIK protein, includes at least one transmembrane domain and at least one P-loop and is encoded by a nucleic acid molecule having a nucleotide sequence which hybridizes under stringent hybridization conditions to a nucleic acid molecule comprising the nucleotide sequence of SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:10, or SEQ ID NO:12.
In another embodiment, the invention features fragments of the proteins having the amino acid sequence of SEQ ID NO: 2, SEQ ID NO:5, SEQ ID NO:8, SEQ ID NO:11, or SEQ ID NO:11, wherein the fragment comprises at least 15 amino acids (e.g., contiguous amino acids) of the amino acid sequence of SEQ ID NO: 2, SEQ ID NO:5, SEQ ID NO:8, SEQ ID NO:11, or an amino acid sequence encoded by the DNA insert of the plasmid deposited with the ATCC as Accession Number PTA-1640. In another embodiment, the protein, preferably a TWIK protein, has the amino acid sequence of SEQ ID NO: 2, SEQ ID NO:5, SEQ ID NO:8, or SEQ ID NO:11.
In another embodiment, the invention features an isolated protein, preferably a TWIK protein, which is encoded by a nucleic acid molecule having a nucleotide sequence at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or more identical to a nucleotide sequence of SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:10, or SEQ ID NO:12, or a complement thereof.
This invention further features an isolated protein, preferably a TWIK protein, which is encoded by a nucleic acid molecule having a nucleotide sequence which hybridizes under stringent hybridization conditions to a nucleic acid molecule comprising the nucleotide sequence of SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:12, or a complement thereof.
The proteins of the present invention or biologically active portions thereof, can be operatively linked to a non-TWIK polypeptide (e.g., heterologous amino acid sequences) to form fusion proteins. The invention further features antibodies, such as monoclonal or polyclonal antibodies, that specifically bind proteins of the invention, preferably TWIK proteins. In addition, the TWIK proteins or biologically active portions thereof can be incorporated into pharmaceutical compositions, which optionally include pharmaceutically acceptable carriers.
In another aspect, the present invention provides a method for detecting the presence of a TWIK nucleic acid molecule, protein or polypeptide in a biological sample by contacting the biological sample with an agent capable of detecting a TWIK nucleic acid molecule, protein or polypeptide such that the presence of a TWIK nucleic acid molecule, protein or polypeptide is detected in the biological sample.
In another aspect, the present invention provides a method for detecting the presence of TWIK activity in a biological sample by contacting the biological sample with an agent capable of detecting an indicator of TWIK activity such that the presence of TWIK activity is detected in the biological sample.
In another aspect, the invention provides a method for modulating TWIK activity comprising contacting a cell capable of expressing TWIK with an agent that modulates TWIK activity such that TWIK activity in the cell is modulated. In one embodiment, the agent inhibits TWIK activity. In another embodiment, the agent stimulates TWIK activity. In one embodiment, the agent is an antibody that specifically binds to a TWIK protein. In another embodiment, the agent modulates expression of TWIK by modulating transcription of a TWIK gene or translation of a TWIK mRNA. In yet another embodiment, the agent is a nucleic acid molecule having a nucleotide sequence that is antisense to the coding strand of a TWIK mRNA or a TWIK gene.
In one embodiment, the methods of the present invention are used to treat a subject having a disorder characterized by aberrant TWIK protein or nucleic acid expression or activity by administering an agent which is a TWIK modulator to the subject. In one embodiment, the TWIK modulator is a TWIK protein. In another embodiment the TWIK modulator is a TWIK nucleic acid molecule. In yet another embodiment, the TWIK modulator is a peptide, peptidomimetic, or other small molecule. In a preferred embodiment, the disorder characterized by aberrant TWIK protein or nucleic acid expression is a CNS disorder.
The present invention also provides a diagnostic assay for identifying the presence or absence of a genetic alteration characterized by at least one of (i) aberrant modification or mutation of a gene encoding a TWIK protein; (ii) mis-regulation of the gene; and (iii) aberrant post-translational modification of a TWIK protein, wherein a wild-type form of the gene encodes a protein with a TWIK activity.
In another aspect the invention provides a method for identifying a compound that binds to or modulates the activity of a TWIK protein, by providing an indicator composition comprising a TWIK protein having TWIK activity, contacting the indicator composition with a test compound, and determining the effect of the test compound on TWIK activity in the indicator composition to identify a compound that modulates the activity of a TWIK protein.
Other features and advantages of the invention will be apparent from the following detailed description and claims.